This invention relates generally to nuclear reactors and more particularly to a lifting, rotating, and sealing apparatus for nuclear reactors utilizing rotating members above the nuclear reactor core.
The nuclear reactor produces heat by fissioning of nuclear material fabricated into fuel elements and assembled within a nuclear core situated in a pressure vessel. In commercial nuclear reactors, heat produced thereby is used to generate electricity. Such nuclear reactors typically comprise one or more primary flow and heat transfer systems, and a corresponding member of secondary flow and heat transfer systems. These secondary heat transfer systems are coupled to conventional steam turbines and electrical generators. A typical energy conversion process for a commercial nuclear reactor, therefore, involves transfer of heat from a nuclear core to a primary coolant flow system, to a secondary coolant flow system, and finally into steam from which electricity is generated.
At the top of the pressure vessel, it is customary to provide a head to seal the nuclear reactor vessel and primary system. A plurality of penetrations pass through the head into the reactor vessel below. These penetrations, typical of which are control rod assemblies and transfer machines, perform functions within the nuclear pressure vessel and the reactor core.
To assure complete access to all areas of the nuclear core, particularly of the liquid metal cooled, breeder type, these penetrations are generally mounted on rotatable members such as rotating plugs. These rotating plugs are cylindrical in nature and of a decreasing size, the innermost plug having the smallest diameter and the outermost plug having the largest diameter. For most efficient operation, each plug is eccentric to the axis of each other plug, and each plug is supported by the next largest plug. The outermost plug is supported by a stationary ring, and is concentric with the ring axis.
During operation, each plug is rotated independently of the other plugs. Additionally, each plug supports penetrations which may rotate independently of the plug.
In conventional nuclear reactors that feature rotating plugs, such as the aforementioned liquid metal cooled reactors, a positive top core holddown is generally not employed. This is because the core holddown structure must be directly over and in contact with the top of the nuclear core during reactor operations, and must be raised prior to rotation during refueling to permit vertical removal of any fuel rod assembly. The difficulty arises because the means of supporting the holddown structure is the rotating plugs themselves. Also, since this rotation and lifting occurs above the nuclear core, all interfaces between the different plugs must be sealed at all times to prevent the escape of any radioactive material.